QGraphicsScene size increases twice at once when an item is movde to its left border...

I have a QGraphicsScene with a few movable items on it. When I drag an item to scene border, the size of the scene increases (it's okay, it should increase), and the scroll bars appear (that's okay too).

But even if the moving item exceeds window borders a few pixels, scene size increases about twice at once, in one big leap.